1. Field of the Invention
The present invention is related to a method of performing an automatic PLMN selection in an IOPS-capable wireless communication system, and more particularly, to a method of performing an automatic PLMN selection in an IOPS-capable wireless communication system for maintaining both normal-mode communications and IOPS mode communications.
2. Description of the Prior Art
The 3rd Generation Partnership Project (3GPP) has developed a universal mobile telecommunications system (UMTS) which adopts a wideband code division multiple access (WCDMA) technology capable of providing high frequency spectrum utilization, universal coverage, and high-speed/quality multimedia data transmission. In the UMTS, a radio access network known as a universal terrestrial radio access network (UTRAN) includes multiple Node-Bs (NBs) for communicating with multiple user equipment (UE). Furthermore, a long-term evolution (LTE) system is now being developed by the 3GPP in order to further improve performance of the UMTS to satisfy users' increasing needs. The LTE system includes a new radio interface and radio network architecture which provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved UTRAN (E-UTRAN) includes multiple evolved NBs (eNBs) for communicating with multiple UEs and a core network (CN) which includes a mobility management entity (MME), a serving gateway (SGW) and other devices for non-access stratum (NAS) control.
In 3GPP Release 13, a study item was studied on isolated E-UTRAN operations for public safety (IOPS) in support of mission critical network operation for public safety (PS). The main idea laid behind this IOPS study item is to investigate needs and requirements for E-UTRAN operations when encountering connection problems with the macro evolved packet core (EPC).
For the scenario that an eNB has no backhaul to the macro EPC, the general IOPS architectural requirements include:                (a) When a UE accesses the eNB in an IOPS mode of operation, a local IP connectivity and transport public safety service shall be provided to the UE by a local EPC connected to the eNB, if authorized.        (b) An eNB that supports IOPS may enter the IOPS mode of operation after it detects lack of S1 connectivity to the macro EPC. The decision by an eNB to enter the IOPS mode of operation shall be made in accordance with the local policies of the radio access network (RAN) operator.        
According to related 3GPP document, the macro EPC is the EPC which serves an eNB in normal mode of operation, while the local EPC is an entity which provides functionality used by an eNB in IOPS mode of operation in order to support public safety services. A nomadic eNB (NeNB) is a nomadic cell which may include a base station, antennas, microwave backhaul and support for local services. The NeNB is intended for PS use by providing coverage or additional capacity where coverage was never present (e.g. forest fire or underground rescue) or where coverage is no longer present (e.g. due to natural disaster). An isolated E-UTRAN can be created by either an E-UTRAN without normal connectivity with the macro EPC or deployed NeNBs with E-UTRAN functionality provided by a local EPC.
FIG. 1 is a diagram illustrating an LOPS architecture of an E-UTRAN 100 in response to an outage event within the network. An isolated E-UTRAN 100A can be created from the E-UTRAN 100 following an event which isolates a part of the E-UTRAN 100 from normal connectivity with the macro EPC or following deployment of standalone E-UTRAN NeNBs within a part of the E-UTRAN 100. The isolated E-UTRAN 100A may include (1) operation with no connection to the macro EPC; (2) one or multiple eNBs; (3) interconnection between eNBs; (4) limited backhaul capacity to the macro EPC; and (5) the services required to support local operations (e.g. group communication) in the case of no network coverage or of limited network coverage.
Referring to FIG. 1, the isolated E-UTRAN 100A can be created when the outage event has been occurred within the wireless communication system. The eNBs in the normal-mode E-UTRAN 100B are connected to the macro EPC by backhaul connection and the macro EPC is connected to the application server. When the isolated E-UTRAN 100A is created, the eNBs in the isolated E-UTRAN 100A are able to support services required for local operations even though a connection to the normal mode E-UTRAN 100B, as well as to the macro EPC, has been cut off.
FIG. 2 is a diagram illustrating UE behavior in an LOPS-capable wireless communication system. For illustrative purpose, it is assumed that a coverage area 20A is served by an LOPS-mode eNB1 (i.e., isolated from the macro EPC) and a coverage area 20B is served by a normal-mode eNB2 (i.e., connected to the macro EPC). PS UE0˜UE5 represent LOPS-capable mobile devices which can support a PS band. The PS UE1˜UE3 may be served by one or multiple cells in the LOPS-mode eNB1 and get connected to each other if the LOPS operation allows it. The PS UE4˜UE5 may be served by one or multiple cells in the normal-mode eNB2 and get connected to each other. The PS UE0 is under the coverage of both the LOPS-mode eNB1 and the normal-mode eNB2. According to 3GPP document TR 23.797 section 6.1, the eNB1 starts advertising an LOPS-dedicated public land mobile network identity (PLMN-ID) in the IOPS mode and only authorized PS UEs can attach to the corresponding IOPS-dedicated PLMN. In the prior art method of performing automatic PLMN selection in the IOPS-capable wireless communication system, UEs are configured to treat this IOPS-dedicated PLMN-ID with lower preference for E-UTRAN access so that other PLMNs of the macro EPC are selected with precedence. In other words, since the PS UE0 is under the coverage of both the IOPS-mode eNB1 and the normal-mode eNB2, the PS UE0 is configured to select the PLMN-ID received from the normal-mode eNB2 and register on the normal-mode eNB2. Under such circumstance, the PS UE0 is able to communication with the PS UE4˜UE5, but lost connection with the PS UE1·UE3.
According to 3GPP document TR 23.797 section 6.1, the user can also perform a manual PLMN selection at any time. For example, the user may select the IOPS-dedicated PLMN-ID received from the IOPS-mode eNB1 for the PS UE0 to register on the IOPS-mode eNB1. Under such circumstance, the PS UE0 is able to communication with the PS UE1˜UE3, but lost connection with the PS UE4˜UE5.
Therefore, there is a need for a method of performing automatic PLMN in an IOPS-capable wireless communication system in order to improve UE's ability to maintain both normal-mode communications and IOPS mode communications.